1. Technical Field
The present relates to a reading circuit for a magnetic-field sensor, for example an anisotropic magnetoresistive (AMR) magnetic sensor, with calibration of the sensitivity of the sensor, and to a corresponding calibration method.
2. Description of the Related Art
Magnetic-field sensors, in particular AMR magnetic sensors, are used in a plurality of applications and systems, for example in compasses, in systems for detecting ferrous materials, in the detection of currents, and in a wide range of other applications, thanks to their capacity of detecting natural magnetic fields (for example, the Earth's magnetic field) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines traversed by electric current).
As it is known, the phenomenon of anisotropic magnetoresistivity occurs within particular ferrous materials, which, when subjected to an external magnetic field, undergo a variation of resistivity as a function of the characteristics of the external magnetic field. Usually, these materials are applied in the form of thin strips so as to form resistive elements, and the resistive elements thus formed are electrically connected to form a bridge structure (typically a Wheatstone bridge).
It is moreover known to manufacture AMR magnetic sensors with standard semiconductor micromachining techniques, as described, for example, in U.S. Pat. No. 4,847,584. In particular, each magnetoresistive element can be formed by a film of magnetoresistive material, such as for example permalloy (i.e., a ferromagnetic alloy containing iron and nickel), deposited to form a thin strip on a substrate made of semiconductor material, for example silicon.
When an electric current is made to flow through a magnetoresistive element, the angle θ between the direction of magnetization of the same magnetoresistive element and the direction of the current flow affect the effective value of resistivity of the magnetoresistive element so that, as the value of the angle θ varies, the value of electrical resistance varies (in detail, the variation follows a law of the cos2θ type). For example, a direction of magnetization parallel to the direction of the current flow results in a maximum resistance value to the passage of current through the magnetoresistive element, whereas a direction of magnetization orthogonal to the direction of the current flow results in a minimum resistance value to the passage of current through the magnetoresistive element.
Usually, AMR magnetic sensors moreover include coils, integrated in the sensors, the so-called “offset straps”, which are designed to generate, when traversed by a current of an appropriate value, a magnetic field that couples in the direction of detection of the sensors; in this regard, see for example U.S. Pat. No. 5,247,278. These offset straps are normally used for operations of compensation of the offsets present in the sensors (on account of mismatches in the values of the corresponding electrical components) and self-test operations. In particular, the value of the electrical quantities at output from the sensors are in this case a function both of the external magnetic field to be detected and of the magnetic field generated internally as a result of the current circulating in the offset straps (which is indeed detected by the magnetoresistive elements). The offset straps are constituted by turns of conductive material, for example metal, which are arranged on the same substrate on which the magnetoresistive elements of the sensor are provided, being electrically insulated from, and set in the proximity of, the same magnetoresistive elements.
In particular, the Wheatstone-bridge detection structure of an AMR magnetic sensor includes magnetoresistive elements having ideally the same resistance value, and such as to form diagonal pairs of equal elements, which react in an opposite way with respect to one another to the external magnetic fields, as shown schematically in FIG. 1 (where I is the electric current flowing in the magnetoresistive elements and R the common resistance value).
If a supply voltage Vs is applied at the input of the bridge detection structure (in particular to first two terminals of the bridge, operating as input terminals), in the presence of an external magnetic field He, a variation of resistance ΔR of the magnetoresistive elements and a corresponding variation of the voltage drop value on the same magnetoresistive elements occur. In fact, the external magnetic field He determines a variation of the direction of magnetization of the magnetoresistive elements. This results in an unbalancing of the bridge, which takes the form of a voltage variation ΔV at output from the bridge circuit (in particular between the remaining two terminals of the bridge, operating as output terminals). Since the direction of the initial magnetization of the magnetoresistive elements is known beforehand, as a function of the voltage variation ΔV it is thus possible to determine the component of the external magnetic field acting in the direction of sensitivity of the magnetic sensor (it is thus possible, using three magnetic sensors with directions of sensitivity orthogonal to one another, to determine the modulus and direction of the external magnetic field).
In particular, in order to detect unbalancing of the Wheatstone bridge and generate an output signal indicating the characteristics of the external magnetic field to be measured, a reading circuit (or front-end) is normally used, which is coupled to the output of the AMR magnetic sensor, and includes, for example, an instrumentation amplifier. The AMR magnetic sensor and the associated reading circuit together form a magnetic-field sensor device, which supplies at output an electrical signal as a function of the detected magnetic field, and has a given input/output response, due in part to the sensitivity of the bridge detection structure, and in part to the gain of the associated reading front-end.
In a known way, the sensitivity of AMR magnetic sensors, i.e., the magnitude of the electrical response supplied by the corresponding bridge detection structure as a function of the external magnetic field to be detected, normally has a high variability (or spread), which can even reach 40% with respect to the nominal value. This spread is due, for example, to the intrinsic process variations associated to the manufacturing of the sensors.
Consequently, a same external magnetic field can generate electrical signals the value of which can vary considerably even between sensors belonging to one and the same production lot. Such a spread in the sensitivity of AMR magnetic sensors is not desirable, in particular in those applications that require an accurate measurement of the magnetic field to be detected, such as, for example, in magnetometers.
Therefore, techniques for calibration of AMR magnetic sensors have been proposed, designed to reduce or at least limit the spread of the sensitivity of the corresponding detection structures.
For example, a calibration technique envisages the use of processes of “laser trimming” during the manufacturing process of the AMR magnetic sensors, i.e., the use of techniques of laser removal for adjusting the values of the electronic components that constitute the sensors. In particular, within an external environment with controlled magnetic field, the electrical characteristics of the sensor are physically adjusted in such a way that it will supply at output an electrical quantity having a value corresponding to that of the external magnetic field, irrespective of the process variations that might have altered the sensitivity thereof.
This technique, however, in addition to being complex and costly to implement (in so far as it requires costly testing and calibration equipment), requires an accurate control of the magnetic field present in the area surrounding the sensors during the calibration operations. This accurate control may, however, prove difficult to achieve on account, for example, of parasitic magnetic fields generated by the testing machinery or coming from the manufacturing environment.
The techniques of calibration of AMR magnetic sensors that have so far been proposed are hence not altogether satisfactory, and frequently are unable to ensure the desired results.